1. Field of the Invention
The present invention relates to a frequency tuning/direct current(DC) offset canceling circuit for continuous-time analog filter with time division, which can tune a cut-off frequency and cancel a DC offset component of a radio frequency(RF) system.
2. Description of the Related Art
In general, the most typical structure for implementing an Radio Frequency system includes a super-heterodyne receiver structure and a direct-conversion receiver structure.
The super-heterodyne receiver is a receiver which performs a signal processing on a baseband by converting an RF signal into an IF frequency and re-converting the resultant frequency into a baseband. The direct-conversion receiver performs a signal processing on a baseband directly without going through an IF frequency.
The direct-conversion receiver has effects of cost reduction, power consumption reduction, entire area reduction due to less uses of analog circuits, whereas it has problems of DC offset, flicker noise, I/O mismatch, and so on.
In particular, noises or offset components of a DC domain are required to be removed because the components cause severe distortion on signals.
Aside from this, when an analog filter in an RF system uses a continuous-time analog filter like an active-RC filter, its cut-off frequency is varied depending on changes of processes, voltages, temperatures, and so on.
FIG. 1 is a circuit diagram showing an integrator-type frequency tuning circuit of differential signaling in a conventional continuous-time analog filter, and FIG. 2 is a circuit diagram of a DC offset canceling circuit of the conventional continuous-time analog filter.
As shown in FIG. 1, an integrator-type frequency tuning circuit of the conventional continuous-time analog filter includes a tuning integrator 11, a comparator 12, and a control signal generator 13.
The tuning integrator 11 includes an operational amplifier used in filters, and frequency characteristics are determined by fixed resistors and variable capacitors. In this case, the variable capacitors include switches controlled by a control signal, and N-bit capacitor array connected to the switches. An integral operation time of the tuning integrator 11 is controlled by a reference clock applied from an outside.
The comparator 12 differentially compares a reference voltage with an output voltage of the integrator per operation period and transfers the resultant voltage to the control signal generator 13.
In this case, when an output voltage difference of the tuning integrator 11 is higher than a reference voltage difference, an output of the comparator 12 becomes “1”. Contrary to this, when an output voltage difference of the tuning integrator 11 is lower than a reference voltage difference, an output of the comparator 12 becomes “0.”
The connection of the capacitor array is controlled by the control signal of the control signal generator 13, thereby determining a size of the variable capacitor.
The frequency tuning circuit may correspond to just one embodiment of the present invention. An active-RC filter may be implemented by using an active-RC type integrator, and a Gm-C filter may be implemented by using a Gm-G type integrator.
A method for acquiring a time constant (RC or gm/C) most closely approximate to the reference voltage within a predetermined period may use a monolithic scheme or a Successive Approximation Register (SAR) scheme.
As shown in FIG. 2, a conventional DC offset canceling circuit of a continuous-time analog filter may use a method for generating a high-pass pole through feedback connection of a signal direction of a primary integrator.
A transfer function of the DC offset canceling circuit of the conventional continuous-time analog filter for generating a high-pass pole through the feedback connection of the signal direction of the primary integrator is defined by equation (1) below.
                              H          ⁡                      (            s            )                          =                                            V              out                                      V              IN                                =                                    K              1                        ⁢                                                            H                  1                                ⁡                                  (                  s                  )                                                            1                +                                                      K                    2                                    ⁢                                                            H                      1                                        ⁡                                          (                      s                      )                                                        ⁢                                                            H                      2                                        ⁡                                          (                      s                      )                                                                                                                              (        1        )            
Herein, H1 denotes a transfer function of a filter, K1, and K2 denote gains of a filter and a feedback path, and H2(s) denotes a transfer function of the feedback path.
In general, the gain of the filter may be represented by “1” in a DC domain, and since it is assumed that the transfer function of the feedback path shows a low-pass characteristic and the gain of the feedback path is “1”, a transfer function in the DC domain may be defined by equation (2) below.
                              H          ⁡                      (            0            )                          =                              1                          1              +                                                H                  2                                ⁡                                  (                  0                  )                                                              =                                    1                              1                -                                                                            ω                      _                                        p                                    s                                                      =                          s                              s                -                                                      ω                    _                                    p                                                                                        (        2        )            
Therefore, a high-pass cut-off frequency in a DC domain becomes ω p of being a low-pass pole of H2(s).
As described above, the continuous-time analog filter used in an RF system is required to be provided with a frequency tuning circuit in order to compensate for frequency change. Aside from this, an RF system is required to be provided with a DC offset canceling circuit in order to cancel noises or offsets of a DC domain.